Make element basically immutable

This isn't *strict* immutabity, which is basically [impossible](http://stackoverflow.com/questions/4828080/how-to-make-an-immutable-object-in-python) in pure python, but it basically means that you can't change elements by accident.

```
In [5]: element.hydrogen.mass += 1*unit.dalton
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-2beb996c81f0> in <module>()
----> 1 element.hydrogen.mass += 1*unit.dalton

AttributeError: can't set attribute
```

If you want to do it, you at least have to try slightly harder
```
In [6]: element.hydrogen._mass += 1*unit.dalton
In [7]: print element.hydrogen._mass
2.007947 Da
```

cc #332